A pressure control of gas is essential for controlling a flow rate of inert gas in semiconductor manufacturing processes, for an air spring vibration isolation table to be used for a base of a semiconductor manufacturing exposure apparatus, for analysis equipments of various kinds of gases, and the like. An accurate pressure control has been required in many fields including apparatuses for semiconductor manufacturing.
Pressure regulators for maintaining a constant pressure which use a mechanical feedback mechanism are available in the market at low prices. For example, an accurate pilot regulator provided with a mechanical feedback mechanism is commonly used. However, if a large amount of gas is flown into the regulator, accurate measurement and control of the gas becomes difficult due to the effect of the gas. Therefore, to avoid the difficulty, a vessel which has a large capacity is installed as a buffer for reducing the effect of the change of the flow rate being used. However, if the vessel having a large capacity is installed, a large space is occupied by the installation.
Therefore, in a pressure control apparatus which controls a pressure in a pressure vessel at a constant value by regulating an amount of air flowing into the pressure vessel by a pneumatic servo valve, inventors of the present invention proposed a pressure control apparatus in the following patent document 1 which measures an inflow rate of air flowing into a pressure vessel by disposing a flow meter at a gas outlet of the pneumatic servo valve and includes a cascade control mechanism consisting of a main loop for feedback-controlling a pressure measured by a pressure gauge by disposing the pressure gauge in the pressure vessel for measuring the pressure in the pressure vessel and a minor loop for feedback-controlling the inflow rate measured by the flow meter. With the technology described above, it has become possible that a pressure of air flowing out through a pneumatic servo valve can be controlled at high speed.
In addition, in the following patent document 2, the inventors of the present invention proposed a vibration isolator provided with an air spring as a supporting leg using a flow rate control servo valve.
Patent document 1: Japanese Patent Laid-open Publication No. 2004-310478 (paragraphs 0016-0021, FIG. 1, FIG. 2)
Patent document 2: Japanese Patent Laid-open Publication No. 2006-144859 (paragraphs 0024-0044, FIG. 10)